// this is mostly based on the s-exp tutorial
// https://github.com/rust-analyzer/rowan/blob/master/examples/s_expressions.rs

use rnix::{match_ast, ast};
use rowan::{GreenNode, GreenNodeBuilder, ast::AstNode};


fn lex(text: &str) -> Vec<(SyntaxKind, String)> {
    fn tok(t: SyntaxKind) -> m_lexer::TokenKind {
        m_lexer::TokenKind(rowan::SyntaxKind::from(t).0)
    }
    fn kind(t: m_lexer::TokenKind) -> SyntaxKind {
        match t.0 {
            0 => L_BRACKET,
            1 => R_BRACKET,
            2 => WORD,
            3 => WHITESPACE,
            4 => ERROR,
            _ => unreachable!(),
        }
    }

    let lexer = m_lexer::LexerBuilder::new()
        .error_token(tok(ERROR))
        .tokens(&[
            (tok(L_BRACKET), r"\["),
            (tok(R_BRACKET), r"\]"),
            (tok(WORD), r"[^\s\[\]]+"),
            (tok(WHITESPACE), r"\s+"),
        ])
        .build();

    lexer
        .tokenize(text)
        .into_iter()
        .map(|t| (t.len, kind(t.kind)))
        .scan(0usize, |start_offset, (len, kind)| {
            let s: String = text[*start_offset..*start_offset + len].into();
            *start_offset += len;
            Some((kind, s))
        })
        .collect()
}


#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[allow(non_camel_case_types)]
#[repr(u16)]
enum SyntaxKind {
    L_BRACKET = 0, // '['
    R_BRACKET,     // ']'
    WORD,          // 'Attrset', 'meta', '.', '>', ...
    WHITESPACE,    // whitespaces is explicit
    ERROR,         // as well as errors

    // composite nodes
    LIST, // `[..]`
    ATOM, // wraps WORD
    ROOT, // top-level (a complete query)
}
use SyntaxKind::*;

impl From<SyntaxKind> for rowan::SyntaxKind {
    fn from(kind: SyntaxKind) -> Self {
        Self(kind as u16)
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
enum Lang {}
impl rowan::Language for Lang {
    type Kind = SyntaxKind;
    fn kind_from_raw(raw: rowan::SyntaxKind) -> Self::Kind {
        assert!(raw.0 <= ROOT as u16);
        unsafe { std::mem::transmute::<u16, SyntaxKind>(raw.0) }
    }
    fn kind_to_raw(kind: Self::Kind) -> rowan::SyntaxKind {
        kind.into()
    }
}

pub struct Parse {
    pub green_node: GreenNode,
    pub errors: Vec<String>,
}

pub fn parse(text: &str) -> Parse {
    struct Parser {
        /// input tokens, including whitespace,
        /// in *reverse* order.
        tokens: Vec<(SyntaxKind, String)>,
        /// the in-progress tree.
        builder: GreenNodeBuilder<'static>,
        /// the list of syntax errors we've accumulated
        /// so far.
        errors: Vec<String>,
    }

    #[derive(Debug)]
    enum QexpRes {
        Ok,
        Eof,
        RBracket,
        LBracket
    }

    impl Parser {
        fn parse(mut self) -> Parse {
            // Make sure that the root node covers all source
            self.builder.start_node(ROOT.into());
            // Parse zero or more S-expressions
            loop {
                match self.word() {
                    QexpRes::Eof => break,
                    QexpRes::Ok => (),
                    unmatched_bracket => {
                        self.builder.start_node(ERROR.into());
                        self.errors.push(format!("lone `{:?}`", unmatched_bracket));
                        self.bump(); // be sure to chug along in case of error
                        self.builder.finish_node();
                    }
                }
            }
            // eat remaining whitespace
            self.skip_ws();
            self.builder.finish_node();

            Parse { green_node: self.builder.finish(), errors: self.errors }
        }
        fn list(&mut self) {
            assert_eq!(self.current(), Some(L_BRACKET));
            // Start the list node
            self.builder.start_node(LIST.into());
            self.bump(); // '['
            loop {
                match self.word() {
                    QexpRes::Eof => {
                        self.errors.push("expected `]`".to_string());
                        break;
                    }
                    QexpRes::RBracket => {
                        self.bump();
                        break;
                    }
                    QexpRes::LBracket => {
                        self.builder.start_node(ERROR.into());
                        self.errors.push("unexpected list".to_string());
                        self.bump();
                        self.builder.finish_node();
                    }
                    QexpRes::Ok => (),
                }
            }
            // close the list node
            self.builder.finish_node();
        }
        fn word(&mut self) -> QexpRes {
            // Eat leading whitespace
            self.skip_ws();
            // Either a list, an atom, a closing paren,
            // or an eof.
            let t = match self.current() {
                None => return QexpRes::Eof,
                Some(R_BRACKET) => return QexpRes::RBracket,
                Some(L_BRACKET) => return QexpRes::LBracket,
                Some(t) => t,
            };
            match t {
                WORD => {
                    self.builder.start_node(ATOM.into());
                    self.bump();
                    self.skip_ws();
                    if Some(L_BRACKET) == self.current() {
                        self.list();
                    }
                    self.builder.finish_node();
                }
                ERROR => self.bump(),
                _ => unreachable!(),
            }
            QexpRes::Ok
        }
        /// Advance one token, adding it to the current branch of the tree builder.
        fn bump(&mut self) {
            let (kind, text) = self.tokens.pop().unwrap();
            self.builder.token(kind.into(), text.as_str());
        }
        /// Peek at the first unprocessed token
        fn current(&self) -> Option<SyntaxKind> {
            self.tokens.last().map(|(kind, _)| *kind)
        }
        fn skip_ws(&mut self) {
            while self.current() == Some(WHITESPACE) {
                self.bump()
            }
        }
    }

    let mut tokens = lex(text);
    tokens.reverse();
    Parser { tokens, builder: GreenNodeBuilder::new(), errors: Vec::new() }.parse()
}

/// To work with the parse results we need a view into the
/// green tree - the Syntax tree.
/// It is also immutable, like a GreenNode,
/// but it contains parent pointers, offsets, and
/// has identity semantics.

type SyntaxNode = rowan::SyntaxNode<Lang>;
#[allow(unused)]
type SyntaxToken = rowan::SyntaxToken<Lang>;
#[allow(unused)]
type SyntaxElement = rowan::NodeOrToken<SyntaxNode, SyntaxToken>;

impl Parse {
    fn syntax(&self) -> SyntaxNode {
        SyntaxNode::new_root(self.green_node.clone())
    }
}

/// Let's check that the parser works as expected
#[test]
fn test_parser() {
    let text = "Inherit > mdDoc[something]";
    let node = parse(text).syntax();
    assert_eq!(
        format!("{:?}", node),
        "ROOT@0..26"
    );
    assert_eq!(node.children().count(), 3);
    let children = node
        .descendants_with_tokens()
        .map(|child| format!("{:?}@{:?}", child.kind(), child.text_range()))
        .collect::<Vec<_>>();

    assert_eq!(
        children,
        vec![
            "ROOT@0..26".to_string(),
             "ATOM@0..8".to_string(),
              "WORD@0..7".to_string(),
              "WHITESPACE@7..8".to_string(), // note, explicit whitespace!
             "ATOM@8..10".to_string(),
              "WORD@8..9".to_string(),
              "WHITESPACE@9..10".to_string(),
             "ATOM@10..26".to_string(),
              "WORD@10..15".to_string(),
              "LIST@15..26".to_string(),
               "L_BRACKET@15..16".to_string(),
                "ATOM@16..25".to_string(),
                 "WORD@16..25".to_string(),
               "R_BRACKET@25..26".to_string()
        ]
    );
}



type NixExprs = Box<dyn Iterator<Item = rnix::SyntaxNode>>;

macro_rules! ast_node {
    ($ast:ident, $kind:ident) => {
        #[derive(PartialEq, Eq, Hash)]
        #[repr(transparent)]
        struct $ast(SyntaxNode);
        impl $ast {
            #[allow(unused)]
            fn cast(node: SyntaxNode) -> Option<Self> {
                if node.kind() == $kind {
                    Some(Self(node))
                } else {
                    None
                }
            }
        }
    };
}

ast_node!(Root, ROOT);
ast_node!(Atom, ATOM);
ast_node!(List, LIST);

// Sexp is slightly different, so let's do it by hand.
#[derive(PartialEq, Eq, Hash, Debug)]
#[repr(transparent)]
struct Qexp(SyntaxNode);

enum QexpKind {
    Atom(Atom),
    List(List),
}

impl Qexp {
    fn cast(node: SyntaxNode) -> Option<Self> {
        if Atom::cast(node.clone()).is_some() || List::cast(node.clone()).is_some() {
            Some(Qexp(node))
        } else {
            None
        }
    }

    fn kind(&self) -> QexpKind {
        Atom::cast(self.0.clone())
            .map(QexpKind::Atom)
            .or_else(|| List::cast(self.0.clone()).map(QexpKind::List))
            .unwrap()
    }

    fn apply(&self, _acc: NixExprs) -> NixExprs {
        todo!()
    }
}

// Let's enhance AST nodes with ancillary functions and
// eval.
impl Root {
    fn qexps(&self) -> impl Iterator<Item = Qexp> + '_ {
        self.0.children().filter_map(Qexp::cast)
    }
}

enum Op {
    Down,
    DownRecursive,
    Up,
    UpRecursive,
    Named(String)
}

impl Atom {
    fn eval(&self) -> Option<i64> {
        self.text().parse().ok()
    }
    fn as_op(&self) -> Option<Op> {
        let op = match self.text().as_str() {
            ">" => Op::Down,
            ">>" => Op::DownRecursive,
            "<" => Op::Up,
            "<<" => Op::UpRecursive,
            name => Op::Named(name.to_owned()),
        };
        Some(op)
    }
    fn text(&self) -> String {
        match self.0.green().children().next() {
            Some(rowan::NodeOrToken::Token(token)) => token.text().to_string(),
            _ => unreachable!(),
        }
    }
    fn apply(&self, acc: NixExprs) -> NixExprs {
        match self.as_op() {
            Some(Op::Down) => Box::new(acc.map(|s| s.children()).flatten()),
            Some(Op::DownRecursive) => Box::new(acc.map(|s| s.descendants()).flatten()),
            Some(Op::Up) => Box::new(acc.filter_map(|s| s.parent())),
            Some(Op::UpRecursive) => Box::new(acc.map(|s| s.ancestors()).flatten()),
            Some(Op::Named(name)) =>
                Box::new(acc
                .filter(move |node| match_ast! { match node {
                    ast::AttrpathValue(value) => {
                        name == value.attrpath().unwrap().to_string()
                    },
                    ast::Apply(value) => {
                        // TODO: special case lambda = NODE_SELECT here?
                        name == value.lambda().unwrap().to_string()
                    },
                    // TODO: this is difficult — I want to use free-form names
                    // to select things below, too, but that might not always be
                    // possible
                    ast::Ident(value) => {
                        name == value.to_string()
                    },
                    _ => false
                }})),
            _ => todo!()
        }
    }
}

impl List {
    fn sexps(&self) -> impl Iterator<Item = Qexp> + '_ {
        self.0.children().filter_map(Qexp::cast)
    }
}


impl Parse {
    fn root(&self) -> Root {
        Root::cast(self.syntax()).unwrap()
    }

    pub fn apply(&self, _content: &str, nexp: rnix::SyntaxNode) -> anyhow::Result<Vec<rnix::SyntaxNode>> {

        let mut acc: NixExprs = Box::new(std::iter::once(nexp));

        for qexp in self.root().qexps() {
            match qexp.kind() {
                QexpKind::Atom(filter) => {
                    acc = filter.apply(acc);
                }
                _ => panic!("???")
            }
        }

        // let results =
        //     acc.map(|node| content[node.text_range().start().into()..node.text_range().end().into()].to_owned())
        //     .collect();

        Ok(acc.collect())
    }
}